Catalysis of hydrogen evolution reaction using ruthenium ion complexed carbon nitride materials

ABSTRACT

A method can include incorporating graphene oxide (GO) in a solution, reducing the graphene oxide (GO) by refluxing carbon nitride (C 3 N 4 ) in the solution to form carbon-nitride refluxed-graphene-oxide (C 3 N 4 -rGO) composites, and incorporating ruthenium ions into the C 3 N 4 -rGO composites to form C 3 N 4 -rGO-Ru complexes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International Application No. PCT/US2018/054148, filed Oct. 3, 2018, which claims priority to and the benefit of U.S. Provisional Patent Application No. 62/568,549 filed Oct. 5, 2017, both of which are incorporated by reference herein in their entirety.

TECHNICAL FIELD

The disclosed technology relates generally to hydrogen evolution reaction (HER), and more particularly to the use of carbon nitride and ruthenium in performing hydrogen evolution reaction.

BACKGROUND

As used herein, the term hydrogen evolution reaction (HER) generally refers to a method for generating hydrogen gas from water. Present approaches to catalyzing this reaction involve the use of high amounts of metallic platinum (Pt). However, platinum is expensive and corrodes easily in acid solutions, which are typically preferred solutions in which to perform the hydrogen evolution reaction. Carbon-based and carbon-nitrogen-based catalysts have been created and may be more stable than platinum but neither of them release hydrogen as efficiently as platinum.

HER generally involves multiple electron-transfer processes and requires appropriate catalysts to achieve a fast hydrogen evolution rate. While carbon-supported Pt has been historically recognized as a leading catalyst for HER because of a high exchange-current density and small Tafel slope, the significant costs of Pt have severely hampered wide-spread applications. Thus, development of non-platinum HER catalysts as cost-effective alternatives has been attracting a great deal of attention. For example, transition metal sulfides, nitrides, phosphides, carbides and oxides (e.g., MX, with M=Mo, Fe, Co, Ni, etc.) have been found to exhibit apparent activity toward HER. However, because of low dispersibility in water, the catalytic activity is limited by the accessibility of the active centers. In addition, the durability may be compromised due to structural instability of the catalysts at low pH, which is a typical condition for HER.

In contrast, for homogeneous catalysts based on organometallic complexes, such as cobalt macrocyclic glyoxime and tetraimine complexes, cobalt and nickel diimine-dioxime complexes, copper phthalocyanine complexes, and ruthenium complexes, surface accessibility is markedly enhanced. In these catalysts, the metal centers are coordinated to nitrogen-containing organic ligands, and the resulting M-N_(x) moieties are generally believed to serve as the active sites for HER catalysis. Yet, despite much progress, the HER performance has remained largely subpar as compared to that of state-of-the art platinum catalysts.

Thus, there remains a need for cost-effective, high-performance electrocatalysts for hydrogen evolution reaction.

SUMMARY

Certain implementations of the disclosed technology generally include the incorporation of ruthenium (Ru) ion, which is advantageously more effective at catalyzing HER than other transition metal ions including iron, cobalt, nickel and copper, into a carbon nitride nanosheet, and subsequent incorporation of graphene oxide into the nanosheet. The resulting composite may catalyze HER about 30% as efficiently as a platinum catalyst but uses a much smaller amount of metal (e.g., 1.9%) and, therefore, is significantly less costly.

In certain embodiments, ruthenium ions may be embedded into the molecular skeletons of graphitic carbon nitride (C₃N₄) nanosheets of 2.0±0.4 nm in thickness by refluxing C₃N₄ and RuCl₃ in water, which takes advantage of the strong affinity of ruthenium ions to pyridinic nitrogen of the tri-s-triazine units of C₃N₄. The formation of C₃N₄—Ru nanocomposites can be confirmed in optical and X-ray photoelectron spectroscopic measurements, which suggests charge transfer from the C₃N₄ scaffold to the ruthenium centers. Significantly, the hybrid materials may be readily dispersible in water, exhibit apparent electrocatalytic activity towards HER in acid and the activity increases with the loading of ruthenium metal centers in the C₃N₄ matrix. In certain embodiments, saturating the sample with ruthenium ion complexation at the atomic ratio of ruthenium to pyridinic nitrogen of ca. 1:2 displays an optimal performance, with an overpotential of only 140 mV to achieve the current density of 10 mA/cm², a low Tafel slope of 57 mV/dec, and a large exchange current density of 0.072 mA/cm². The activity is markedly lower when C₃N₄ is embedded with other metal ions such as Fe³⁺, Co³⁺, Ni³⁺ and Cu²⁺, which suggests a minimal contribution from the C₃N₄ nanosheets to the HER activity. The HER activity is most likely due to the formation of Ru—N moieties where the synergistic interactions between the carbon nitride and ruthenium metal centers facilitated the adsorption of hydrogen.

Carbon-based materials are promising electrocatalysts toward hydrogen evolution reaction (HER) due to their low costs; yet the catalytic performance needs to be further improved before commercialization. In certain embodiments, reduced graphene oxide is incorporated into a ruthenium ion-complexed carbon nitride sheet forming C₃N₄-rGO-Ru composites through Ru—(N═C)₂ coordination at a low Ru loading. The incorporation of both Ru ions and rGO into the composite generally leads to an electron redistribution of the materials, which may significantly enhance the HER performance as compared to other carbon-based HER electrocatalysts, with an overpotential of −80 mV for a 10 mA/cm² current density, Tafel slope 55 mV/dec, exchange current density 0.462 mA/cm², for example. This may be ascribed to the positive shift of the conduction band of the composite where the charge carrier density increased markedly by more than 200 times as compared to that of C₃N₄, leading to a lower energy barrier of hydrogen evolution and enhanced reaction kinetics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an exemplary schematic structure of C₃N₄—Ru nanosheets.

FIG. 1B illustrates a representative TEM image of C₃N₄—Ru—F.

FIGS. 1C-F respectively illustrate EDX images of carbon (C), nitrogen (N), ruthenium (Ru), and chlorine (Cl). in C₃N₄—Ru—F (scale bars all 50 nm).

FIG. 1G illustrates a representative AFM topograph of C₃N₄—Ru—F (scale bar 200 nm).

FIG. 1H illustrates an exemplary height profile of the line scan in FIG. 1G.

FIG. 1I illustrates an exemplary histogram of the thickness of C₃N₄—Ru—F nanosheets based on the AFM topographic measurements.

FIG. 2A illustrates an exemplary XPS survey spectra of (i) C₃N₄, (ii) C₃N₄—Ru—P, (iii) C₃N₄—Ru—F.

FIG. 2B illustrates an exemplary high-resolution XPS spectra of Cl 2p electrons.

FIG. 2C illustrates an exemplary high-resolution XPS spectra of C 1s and Ru 3d electrons.

FIG. 2D illustrates an exemplary high-resolution XPS spectra of N 1s electrons.

FIG. 3A illustrates an example of ultraviolet (UV)-vis spectra of C₃N₄ (black curve), C₃N₄—Ru—F (red curve) and the difference between these two spectra (green curve).

FIG. 3B illustrates an example of photoluminescence spectra of C₃N₄ (black curve) and C₃N₄—Ru—F (red curve). Inset are the corresponding photographs of C₃N₄ and C₃N₄—Ru dispersions under 365 nm UV irradiation.

FIG. 4A illustrates an example of polarization curves of HER on various electrocatalysts in 0.5 M H₂SO₄.

FIG. 4B illustrates an example of corresponding Tafel plots derived from FIG. 4A.

FIG. 4C illustrates exemplary cyclic voltammograms within the range of +0.1 to +0.2 V where no faradaic reaction occurred at difference scan rates.

FIG. 4D illustrates an exemplary variation of the double-layer charging currents at +0.15 V versus scan rate.

FIG. 4E illustrates an example of Nyquist plots collected at the overpotential of −50 mV. Inset is the equivalent circuit of the electrocatalyst-modified electrode, where R_(s) is (uncompensated) resistance, R_(ct) is charge-transfer resistance and CPE is constant-phase element (equivalent to C_(dl)).

FIG. 4F illustrates an example of the 1^(st) and 1000th cycle of HER polarization curves on C₃N₄—Ru—F in a stability test in accordance with certain implementations of the disclosed technology.

FIG. 5A illustrates exemplary band structures of C₃N₄.

FIG. 5B illustrates exemplary projected density of states of C₃N₄.

FIG. 5C illustrates exemplary band structures of C₃N₄—Ru.

FIG. 5D illustrates exemplary projected density of states of C₃N₄—Ru, where contributions of Ru 4d and 5 s orbitals to the PDOS are labeled in different colors.

FIG. 5E illustrates an example of calculated Gibbs free-energy (ΔG_(H*)) of HER at the equilibrium potential for C₃N₄ (blue) and C₃N₄—Ru (red) at various bonding sites as labeled in the panel inset.

FIG. 5F illustrates an exemplary schematic of interfacial charge transfer in C3N4-Ru, where red signals represent positive charge and blue signals represent negative charge with an isosurface value of 0.003 e/au³ in accordance with certain implementations of the disclosed technology.

FIG. 6 illustrates an exemplary comparison of exchange current densities of the as-compared electrocatalyst in accordance with certain implementations of the disclosed technology.

FIG. 7 illustrates an exemplary comparison of current densities of the as-compared electrocatalyst under the overpotential of −150 mV in accordance with certain implementations of the disclosed technology.

FIG. 8 illustrates exemplary representative Nyquist plots of C₃N₄-rGO-Ru complexes at varieties of overpotentials, where the electron transfer resistances are 46.4, 19.7, 9.1 and 5.0Ω at −50, −100, −150 and −200 mV overpotentials, respectively.

FIG. 9a illustrates exemplary CV curves of C₃N₄ within the range of 0.1 V to 0.2 V where no faradaic reaction occurred at different scan rates in accordance with certain implementations of the disclosed technology.

FIG. 9b illustrates exemplary CV curves of C₃N₄-rGO within the range of 0.1 V to 0.2 V where no faradaic reaction occurred at different scan rates in accordance with certain implementations of the disclosed technology.

FIG. 10 illustrates an example of synthetic processes of the C₃N₄-rGO-Ru complex in accordance with certain implementations of the disclosed technology.

FIGS. 11a-c illustrate representative AFM topographes of (a ₁) C₃N₄, (b ₁) C₃N₄-rGO composites and (c ₁) C₃N₄-rGO-Ru complex, and the corresponding height profiles (a ₂ , b ₂, and c ₂) of the lines scans, and thickness histograms (a ₃ , b ₃, and c ₃) of the thickness.

FIGS. 11d-e illustrate exemplary scanning TEM images and the corresponding elemental maps of carbon, nitrogen and ruthenium of (d) C₃N₄-rGO composites and (e) C₃N₄-rGO-Ru complexes.

FIG. 12a illustrates XPS survey spectra of (i) C₃N₄, (ii) C₃N₄-rGO, and (iii) C₃N₄-rGO-Ru.

FIG. 12b illustrates high-resolution XPS spectra of C 1s and Ru 3d.

FIG. 12c High-resolution XPS spectra of N 1s.

FIG. 13a illustrates exemplary polarization curves of HER on various electrocatalysts in 0.5 M H₂SO₄.

FIG. 13b illustrates corresponding Tafel plots derived from panel (a).

FIG. 13c illustrates an exemplary comparison of the HER performance of the disclosed electrocatalyst to prior carbon based electrocatalysts.

FIG. 13d illustrates exemplary Nyquist plots collected at the overpotential of −100 mV. Inset is the equivalent circuit of the electrocatalyst-coated electrode, where R_(s) is uncompensated resistance, R_(ct) is charge transfer resistance and CPE is constant-phase element (e.g., equivalent to C_(dl)).

FIG. 14 illustrates an exemplary comparison of exchange current densities of the as-compared electrocatalyst in accordance with certain implementations of the disclosed technology.

FIG. 15 illustrates an exemplary comparison of current densities of the as-compared electrocatalyst under the overpotential of −150 mV in accordance with certain implementations of the disclosed technology.

FIG. 16 illustrates representative Nyquist plots of C₃N₄-rGO-Ru complexes at varieties of overpotentials. In the example, the electron transfer resistances are 46.4, 19.7, 9.1 and 5.0Ω at −50, −100, −150 and −200 mV overpotentials, respectively.

FIG. 17a illustrates exemplary cyclic voltammograms within the range of 0.1 V to 0.2 V where no faradaic reaction occurred at different scan rates.

FIG. 17b illustrates variation of the double-layer charging currents at 0.15 V versus scan rate.

FIG. 17c illustrates exemplary Nyquist plots of electrocatalysts without carbon black collected in 0.5 M H₂SO₄ electrolyte at the overpotentials of −100 mV. Inset is the full plot of C₃N₄.

FIG. 17d illustrates exemplary Mott-Schottky plots of electrocatalysts collected in 0.1 M Na₂SO₄ electrolyte at a frequency of 1000 Hz. In the example, the potential is versus Ag/AgCl reference electrode.

FIG. 18 illustrates exemplary CV curves of (a) C₃N₄ and (b) C₃N₄-rGO within the range of 0.1 V to 0.2 V where no faradaic reaction occurred at different scan rates.

FIG. 19 illustrates exemplary HER current density of C₃N₄—Ru—F, C₃N₄—Ru—P, and C₃N₄ at the overpotential of −200 mV.

FIG. 20 illustrates exemplary XPS spectra of the C₃N₄-M nanocomposites (M=Fe³⁺, Co³⁺, Ni³⁺ and Cu²⁺): (A) Fe 2p, (B) Co 2p, (C) Ni 2p and (D) Cu 2p, where the black curves are experimental data and the colored curves are deconvolution fits.

FIG. 21 illustrates exemplary HER performance of the C₃N₄-M nanocomposites (M=Fe²⁺, Co²⁺, Ni²⁺ and Cu²⁺). As indicated in the figure legends, the overpotentials (η₁₀) needed to reach the current density of 10 mA/cm² were all very close to that of C₃N₄ alone and much more negative than that of C₃N₄—Ru—F.

FIG. 22A is an exemplary schematic illustration of the full water splitting cell setup.

FIG. 22B is a photograph of the home-made water splitting setup.

FIG. 22C illustrates exemplary results of water displacement during 20 min of electrochemical operation. Inset is the photograph of water displacement after 20 min.

FIG. 23 illustrates exemplary Nyquist plots of HER on C₃N₄—Ru—F at different overpotentials (i.e., −50, −100, −150 and −200 mV).

FIG. 24 illustrates exemplary XPS spectra of the C 1s and Ru 3d electrons in C₃N₄—Ru—F before and after 1000 electrochemical cycles.

FIG. 25A illustrates exemplary 1×1 cell structures of C₃N₄.

FIG. 25B illustrates exemplary 2×2 cell structures of C₃N₄.

FIG. 25C illustrates exemplary 1×1 cell structures of C₃N₄—Ru.

FIG. 25C illustrates exemplary 2×2 cell structures of C₃N₄—Ru.

FIGS. 26A and 26B respectively illustrate exemplary stable hydrogen adsorption on N and C of C₃N₄ after relaxing calculations, where the tops represent topic view and bottoms represent side view.

FIGS. 27A, 27B, and 27C respectively illustrate exemplary stable hydrogen adsorption on Ru, N1, and C of C₃N₄—Ru after relaxing calculations, where the tops represent topic view and bottoms represent side view.

DETAILED DESCRIPTION

In certain implementations, HER electrocatalysts include the thermal refluxing of graphitic C₃N₄ nanosheets and RuCl₃ in water, leading to the formation of C₃N₄—Ru hybrids that exhibit apparent HER activity in acidic media. The HER activity may increase with increased loading of ruthenium ions in the C₃N₄ matrix, and in certain examples samples may display an overpotential of only −140 mV to achieve the current density of 10 mA/cm², a Tafel slope of 57 mV/dec, and an exchange current density of 0.072 mA/cm², which is superior to results reported in recent literatures with relevant HER electrocatalysts. This remarkable performance may be attributed to the formation of Ru—N₂ moieties that facilitate the adsorption of hydrogen, a critical step in HER catalysis, as confirmed by studies based on DFT calculations. Graphitic C₃N₄ nanosheets may be exploited as a unique functional scaffold for the fabrication of a wide range of single atom-like catalysts for diverse applications.

By taking advantage of the abundant pyridinic nitrogen moieties in graphitic carbon nitride (C₃N₄) nanosheets, ruthenium ions may be embedded within the C₃N₄ molecular skeleton, forming Ru—N_(x) moieties that may serve as effective active sites for HER, analogous to conventional organometallic complexes. With high chemical/thermal stability, C₃N₄ has been explored as advanced metal-free catalysts for a variety of energy conversion/storage processes. For instance, C₃N₄-based materials have been used as effective photocatalysts where the electronic band gap structure may be readily manipulated by doping with non-metal elements. However, applications of C₃N₄ in electrocatalysis, such as HER, have been limited by the low electrical conductivity. This may be mitigated by the incorporation of metal ions into the C₃N₄ molecular skeleton by taking advantage of the tri-s-triazine units of C₃N₄ that readily chelate transition metal ions.

Incorporation of ruthenium metal ions into the C₃N₄ matrix may be manifested by the emergence of unique metal-ligand charge transfer (MLCT) in UV-vis and photoluminescence (PL) measurements. XPS measurements may suggest electron transfer from the C₃N₄ skeleton to the ruthenium metal centers. Remarkably, the C₃N₄—Ru nanocomposites can be readily dispersed in water and exhibit apparent HER activity in acid, which increases with increasing loading of the ruthenium metal centers. Saturating a sample with ruthenium complexation at a ruthenium to pyridinic nitrogen ratio of ca. 1:2 may display an optimal performance, e.g., with a low overpotential of only −140 mV to achieve the current density of 10 mA/cm², a Tafel slope of 57 mV/dec, and an exchange current density of 0.072 mA/cm², which is superior to results reported recently with C₃N₄-based HER catalysts. This performance might be the result of the formation of Ru—N moieties where the synergistic interactions between pyridinic nitrogen and ruthenium metal centers facilitate the adsorption of protons with a decrease of the Gibbs free energy.

Graphitic C₃N₄ nanosheets may be synthesized by thermal treatment of melamine in air. For example, in certain implementations, 10 g of melamine may be placed in a ceramic crucible with a cover and heated to 600° C. at a heating rate of 2.3° C./min. The sample may be heated at this temperature for 3 hours before being cooled down to room temperature, yielding a product that may be ground into a powder. To synthesize ruthenium ion-complexed carbon nitride (C₃N₄—Ru), 50 mg of the C₃N₄ powder synthesized above may be first dispersed into 50 mL of Nanopure H₂O under sonication overnight to produce C₃N₄ thin layers. 56 mg of RuCl₃ may be added into the mixture, which may be refluxed for 4 hours. The product may be collected by centrifugation at 4500 rpm for 10 minutes and washed with Nanopure H₂O and ethanol to remove excess ruthenium ions, for example. In this synthesis, the supernatant may show a light brown color, indicating that there is a small excess of ruthenium ions in the solution and C₃N₄ is saturated with ruthenium complexation. The corresponding sample may be referred to as C₃N₄—Ru—F. Another sample may be prepared in the same manner except the amount of RuCl₃ added may be reduced by half to 28 mg. After centrifugation, the supernatant may be colorless, indicating that all ruthenium ions are incorporated into C₃N₄. The product may be denoted as C₃N₄—Ru—P.

C₃N₄ complexed with other transition-metal ions (e.g., Fe³⁺, Co³⁺, Ni³⁺ and Cu²⁺) may also be prepared in a similar fashion where an equivalent amount of salt precursors may be used instead of RuCl₃, and the corresponding products may be referred to as C₃N₄—Fe, C₃N₄—Co, C₃N₄—Ni, and C₃N₄—Cu. For these samples, the supernatants after centrifugation may show the same colors as those of the original metal salts, suggesting that the metal ions are in excess and C₃N₄ is saturated with the respective metal ions.

In certain implementations, transmission electron microscopic (TEM) measurements were performed with a JOEL JEM 2100F microscope, atomic force microscopic (AFM) measurements were carried out with a Molecular Imaging PicoLE SPM instrument, X-ray diffraction (XRD) patterns were acquired with a Rigaku Americas Miniflex Plus powder diffractometer operated at the voltage of 40 kV and current of 30 mA, XPS measurements were carried out with a PHI 5400/XPS instrument equipped with an Al K_(α) source operated at 350 W and 10⁻⁹ Torr, UV-vis spectra were collected with a Perkin Elmer Lambda 35 UV-vis spectrometer, and PL measurements were performed with a PTI fluorospectrometer. Inductively coupled plasma mass spectrometric (ICP-MS) analysis was carried out with an Agilent 1260-7700e instrument.

In these implementations, electrochemical tests were performed using a CHI710 workstation and electrochemical impedance measurements were carried out with a Gamry Reference 600 instrument. A Ag/AgCl electrode (e.g., saturated KCl) and Pt wire were used as the reference electrode and counter electrode, respectively, while a glassy carbon electrode (5 mm in diameter, 0.196 cm²) was used as the working electrode. The Ag/AgCl electrode was calibrated against a reversible hydrogen electrode (RHE) and all the potentials were referred to this RHE electrode. To prepare catalyst inks, 2 mg of C₃N₄-M powders and 3 mg of carbon black were dispersed in 1 mL of a 1:4 (v:v) water/ethanol mixed solvents along with 10 μL of a Nafion solution, and the mixture was sonicated for 30 minutes to achieve good dispersion of the materials. Then 15 μL of the inks was dropcast onto the surface of the glassy carbon electrode and dried at room temperature, corresponding to a mass loading of 0.153 mg/cm² for the catalysts.

In these implementations, full water splitting was carried out with C₃N₄—Ru—F as the HER catalyst and commercial RuO₂ as the catalyst for oxygen evolution reaction (OER), along with a Ag/AgCl reference electrode. To prepare the electrodes, the catalysts were dispersed in ethanol at a concentration of 2 mg/mL under sonication for 0.5 h; then 0.5 mL of the catalyst inks was dropcast onto a piece of carbon cloth (1 cm×2 cm) yielding a mass coverage of 0.5 mg/cm². Water splitting tests were performed with an applied potential of 2 V in 1 M KOH, and the amounts of hydrogen and oxygen generated were quantified by water displacement measurements.

In these implementations, the calculations of the electronic structures of C₃N₄ and C₃N₄—Ru were carried out by using open-source planewave code, Quantum Espresso. The two-dimensional unit cell was built with two chemical formula of C₃N₄ and one Ru atom. The interlayer distance was set at 20 Å so that there was no interaction between the layers. The ultrasoft pseudo-potential was adopted with the wavefunction cutoff of 40 Ry (e.g., charge density cutoff 200 Ry), the energy threshold at 10⁻⁸ Ry, and the force converged to 10⁻⁴ a.u. The Marzari-Vanderbilt smearing was adopted with 0.01 Ry for C₃N₄—Ru. The K point was set at 4×4×1. The vibration frequencies of surface species and zero-point energy (ZPE) and entropy contribution were evaluated by density functional perturbation theory (DFPT). All atoms were initiated with spin polarization.

In certain implementations, C₃N₄ nanosheets were synthesized by thermal treatment of melamine in air, and refluxing with RuCl₃ in water led to effective incorporation of ruthenium metal ions into the C₃N₄ scaffolds, forming Ru—N_(x) moieties through the pyridinic nitrogen, as schematically illustrated by FIG. 1A. FIG. 1B depicts a typical TEM image of the C₃N₄—Ru—F sample where nanosheet structures of a few tens of nm can be readily identified, which consistent with the as-prepared C₃N₄ (see FIG. 6). In XRD measurements (see FIG. 7), both C₃N₄—Ru—F and as-prepared C₃N₄ nanosheets displayed a single diffraction peak centered at 27.2°, corresponding to an interplanar spacing of 0.326 nm that is characteristic of the C₃N₄ (002) planes. Energy dispersive x-ray (EDX) measurements confirmed that Ru ions were incorporated into the C₃N₄ matrix, as manifested in the elemental maps of C, N, Ru and Cl in FIGS. 1C-F, which were all distributed rather evenly across the sample.

A representative AFM topograph is illustrated by FIG. 1G, and the height profile of a line scan is illustrated by FIG. 1H, where the thickness of the C₃N₄—Ru nanosheets is generally consistent at ca. 2 nm. Statistical analysis based on more than 100 nanosheets showed that the average thickness was 2.0±0.4 nm, as manifested in the thickness histogram (see FIG. 1I), which is identical to that of the as-produced C₃N₄ nanosheets (see FIG. 8).

In these implementations, XPS measurements were then carried out to determine the chemical composition and valence states of the composites. FIG. 2A depicts the survey spectra of (i) C₃N₄, (ii) C₃N₄—Ru—P and (iii) C₃N₄—Ru—F, where the C 1s and N 1s electrons can be readily identified at ca. 285 eV and ca. 399 eV for all samples, and both C₃N₄—Ru—F and C₃N₄—Ru—P also exhibited two additional peaks at ca. 282 eV and ca. 199 eV, due to Ru 3d and Cl 2p electrons, respectively. The high-resolution scan of the Cl 2p electrons is illustrated by FIG. 2B, where the binding energy was found to peak at 197.50 (Cl 2p_(3/2)) and 199.00 eV (Cl 2p_(1/2)) for C₃N₄—Ru—P and 197.70 (Cl 2p_(3/2)) and 199.20 eV (Cl 2p_(1/2)) for C₃N₄—Ru—F, consistent with those of Cl⁻ ions in outer-sphere. The C 1s and Ru 3d spectra are illustrated by FIG. 2C. For the as-prepared C₃N₄, the C 1s spectrum can be deconvoluted into two peaks, a major one at 287.31 eV and a minor one 284.06 eV. The former may be assigned to the sp²-hybridized carbon in N—C═N of the C₃N₄ matrix, while the latter likely arose from defective carbon in sp³ C—C bonds. Interestingly, the binding energy of C 1s in N—C═N blue-shifted somewhat to 287.57 eV for C₃N₄—Ru—P and even further to 287.93 eV for C₃N₄—Ru—F, likely due to the binding of (positively charged) ruthenium ions to the nitrogen moiety. For the Ru 3d electrons, the doublet can be resolved at 281.67 eV (Ru 3d_(5/2)) and 285.77 eV (Ru 3d_(3/2)) for C₃N₄—Ru—P, and slightly lower at 281.30 eV (Ru 3d_(5/2)) and 285.40 eV (Ru 3d_(3/2)) for C₃N₄—Ru—F. These binding energies are close to those of Ru(II) 3d electrons in ruthenium tris-bipyridine complex, indicating that ruthenium was reduced to +2 from the original +3 charge state likely by hydroxide species, and incorporated into the C₃N₄ matrix by Ru—N coordination bonds that enhanced electron-withdrawing of the nitrogen atoms. FIG. 2D illustrates that consistent results can be obtained in the high-resolution scans of the N 1s electros. For the C₃N₄ nanosheets, two peaks were resolved, a major one at 397.80 eV that may be attributed to the sp²-hybridized pyridinic nitrogen (C—N═C) and a minor one at 399.58 eV that can be assigned to the sp³-hybridized tertiary nitrogen (N—(C)₃). After ruthenium ion complexation, the C—N═C peak blue-shifted to 398.08 eV for C₃N₄—Ru—P and 398.48 eV for C₃N₄—Ru—F (whereas the N—(C)₃ peak remained almost invariant). These observations suggest charge transfer from the C₃N₄ skeleton to the Ru d-orbital. Such MLCT may have significant implication in the electrocatalytic activity.

Based on the integrated peak areas, the elemental compositions of the samples in these implementations were then analyzed. First, the atomic ratio of C(N—C═N):N was estimated to be 1:1.27 for C₃N₄, 1:1.20 for C₃N₄—Ru—P, and 1:1.18 for C₃N₄—Ru—F, which is in general agreement with the expected value of 1:1.33. In addition, the Ru to pyridinic N (C═N—C) ratio was estimated to be 1:4.7 for C₃N₄—Ru—P and almost doubled to 1:2.0 for C₃N₄—Ru—F (see Table 1 below). Consistent results were obtained in ICP-MS measurements where the ruthenium content was found to increase with the amount of RuCl₃ added (see FIG. 9). This suggested that in the saturated structure, each Ru center was coordinated to two pyridinic nitrogen sites, as shown in the schematic diagram of FIG. 1A. In addition, the Ru:Cl ratios in both samples were very close at 1:0.5. The Ru centers were not fully coordinated, which may be advantageous for catalytic reactions.

TABLE 1 Atomic percentages of each elements in the materials based on XPS measurements. C N O Ru C₃N₄ 42.14% 53.86% 4.00% — C₃N₄-rGO 61.67% 16.65% 21.68% — C₃N₄-rGO-Ru 67.88% 7.48% 22.71% 1.93%

Further structural insights were obtained in UV-vis and PL measurements in these implementations. FIG. 3A provides that the as-prepared C₃N₄ nanosheets (i.e., black curve) exhibited an absorption peak at around 320 nm, which is similar to that observed with graphene quantum dots, and a weak, broad peak at ca. 390 nm which can be assigned to π-π* transitions that are commonly observed in conjugated ring systems including heterocyclic aromatics. Similar optical characteristics can be seen with C₃N₄—Ru—F (i.e., red curve). However, the difference between these two spectra shows a new absorption band between 350 nm and 550 nm, where the peak at ca. 431 nm (i.e., green curve) is likely due to MLCT transitions.

In these implementations, both the C₃N₄ and C₃N₄—Ru—F exhibited consistent PL profiles, with the emission peak (λ_(em)) at 436 nm under the excitation (λ_(ex)) of 330 nm at room temperature (see FIG. 3B). However, the emission intensity of C₃N₄—Ru—F decreased by about 91% as compared to that of C₃N₄, most likely because the emission coincided with the MLCT absorption of the Ru—N moieties (see panel A) as well as marked diminishment of the C₃N₄ bandgap upon ruthenium ion complexation, as suggested in DFT calculations. This is also manifested in the photographs of C₃N₄ and C₃N₄—Ru—F solutions under photoirradiation at 365 nm (see FIG. 3B inset). The substantial quenching of the PL emission of C₃N₄—Ru—F, as compared to C₃N₄, suggested that ruthenium ion complexation suppressed radiative recombination of the photo-generated electron-hole pairs.

The electrocatalytic activities of the samples towards HER were then evaluated by electrochemical measurements in a N₂-saturated 0.5 M H₂SO₄ solution. FIG. 4A depicts the polarization curves of the various electrocatalysts loaded onto a glassy carbon electrode. The figure illustrates that, at increasingly negative potentials, nonzero currents started to emerge with C₃N₄, C₃N₄—Ru—P and C₃N₄—Ru—F, in comparison to the electrocatalytically inactive carbon black, indicating effective HER activity of the materials. Yet the activity varied markedly among the series. For instance, the overpotential (η₁₀) required to achieve the current density of 10 mA/cm² was only −140 mV for C₃N₄—Ru—F, which is markedly lower than those for C₃N₄—Ru—P (−189 mV) and C₃N₄ (−296 mV). For comparison, η₁₀ for Ru nanoparticles was −233 mV,⁶³ signifying limited activity of (metallic) ruthenium nanoparticles towards hydrogen evolution. This indicates that it is the incorporation of ruthenium metal ions into the C₃N₄ matrix that significantly enhanced the HER activity, which increased with increasing loading of the metal centers (see FIG. 9). Also, at the same overpotential of −200 mV, the current densities were the highest at 33.32 mA/cm² for C₃N₄—Ru—F, as compared to 12.73 mA/cm² for C₃N₄—Ru—P and 2.84 mA/cm² for C₃N₄ (see FIG. 19). That is, the HER activity of C₃N₄—Ru—F is 2.6 times that of C₃N₄—Ru—P and 11.7 times that of C₃N₄. Notably, whereas the overall performance remains subpar as compared to that of state-of-the art Pt/C (which exhibited an η₁₀ of only −38 mV), it is better than leading results of C₃N₄-based HER electrocatalysts reported in recent literature (see Table 2 below, where the first row corresponds to results from the disclosed embodiments and the subsequent rows (labelled a-n) correspond to prior results).

TABLE 2 The HER electrocatalytic performance of the disclosed C₃N₄-rGO-Ru complexes with the carbon-based electrocatalysts. Tafel η₁₀ slope J₀ Loading C_(dl) catalyst (mV) (mV/dec) (μA/cm²) (mg/cm²) (mF/cm²) −80 56 462 0.153 27.8 a BCN −70 100 51 0.943 0.108 b PCN/NG −80 49.1 430 0.57 N/A c S, N—C −116 68 N/A 0.285 27.4 d Co—C—N −138 55 N/A N/A 400 e C₃N₄—Ru −140 57 72 0.153 18.4 f N—P—C −163 89 160 0.159 N/A g CN-nrG −207 54 39.8 0.143 N/A h C₃N₄@G −219 53 49 0.142 10.6 i p-WMCNT −220 71.3 16 0.36 N/A j NENU −237 96 36 N/A N/A k C₃N₄-NG −240 51.5 0.35 0.1 5.0 l NS-500 −276 81 8.4 N/A N/A m C₃N₄—Cu −390 76 N/A 0.28 N/A n p-N, P-G −420 91 0.24 0.204 10.6

Also in these implementations, the linear portions of the polarization curves (see FIG. 4B) were fitted to the Tafel equation, η=a log |j|+b (where j is the current density, and a is the Tafel slope), and the Tafel slope was estimated to be 57 mV/dec for C₃N₄—Ru—F, which is much lower than those for C₃N₄—Ru—P (81 mV/dec) and C₃N₄ (178 mV/dec). For comparison, the Tafel slope was ca. 31 mV/dec for commercial Pt/C, and markedly greater at 234 mV/dec for the poorly active carbon black.

HER involves the following three reactions, each of which carries a specific Tafel slope:

Volmer reaction (Tafel slope 120 mV/dec): H₃O⁺+e⁻→H*+H₂O

Heyrovsky reaction (Tafel slope 40 mV/dec): H*+H₃O⁺+e⁻→H₂+H₂O

Tafel reaction (Tafel slope 30 mV/dec): 2H*→H₂

where the asterisks denote surface-adsorbed species. Based on the Tafel slopes obtained above, the rate-determining step (RDS) of HER on Pt/C is most likely the Tafel reaction where molecular hydrogen (H₂) is formed by adsorbed hydrogen (H*) and released from the catalyst surface, due to the high activity of platinum in the reduction of proton to atomic hydrogen. For the C₃N₄—Ru complexes, the HER activity was likely dictated by the combined Volmer and Heyrovsky reactions, where the RDS involves the formation of metal-hydride moieties. Furthermore, the exchange current density (J_(o)) can be estimated by extrapolation of the Tafel plot to the x axis to be 0.072 mA/cm² for C₃N₄—Ru—F, which is also superior to results reported in recent literatures with relevant electrocatalysts (see Table 2 above). For comparison, J_(o) was markedly lower at 0.014.5 mA/cm² for C₃N₄—Ru—P, and 0.00015 mA/cm² for C₃N₄, whereas much higher at 1.5 mA/cm² for Pt/C.

In these electrochemical measurements, the HER activity of the as-prepared C₃N₄ nanosheets alone was very poor, markedly lower than those of the C₃N₄—Ru composites. This suggests minimal contributions of pyridinic nitrogen in C₃N₄ to hydrogen reduction, in contradiction to results in prior studies where DFT calculations and experimental results suggested that hybrid materials based on carbon nitride and nitrogen-doped graphene might be active for HER electrocatalysis. In the disclosed embodiments, the remarkable HER performance of C₃N₄—Ru is attributed to the ruthenium metal centers embedded within the C₃N₄ matrix, where the Ru—N moieties behaved analogously to conventional metal complexes for HER. In embodiments where the ruthenium metal centers were replaced by other transition-metal ions, such as Fe(II), Co(II), Ni(II), and Cu(II), the HER performance of the resulting C₃N₄-M composites diminished substantially and became comparable to that of C₃N₄ alone (see FIGS. 20 and 21), thus confirming that the unique role of ruthenium centers in the electroreduction of protons to hydrogen. The HER activity of C₃N₄—Ru—F was also manifested in full water splitting with commercial RuO₂ as the OER catalyst, where the amount of hydrogen generated was 2.05 times that of oxygen (see FIG. 22).

Further insights into these interactions between ruthenium metal centers and C₃N₄ matrix were obtained by quantitative analysis of the electrochemical active surface area (ECSA) and charge-transfer resistance (R_(ct)). FIG. 4C illustrates the cyclic voltammograms of C₃N₄—Ru—F recorded at different scan rates (10 to 60 mV/s) in the potential range of +0.1 to +0.2 V vs. RHE, where no faradaic reaction occurred. FIG. 4D illustrates that the current density at +0.15 V versus potential scan rate and the double layer capacitance (C_(dl), which is proportional to ECSA) of C₃N₄—Ru—F was estimated to be 18.4 mF/cm², which was 4.7 times that of C₃N₄ (3.9 mF/cm²), and 1.7 times that of C₃N₄—Ru—P (10.7 mF/cm²). This may be attributed to the enhanced electrical conductivity of the composites with the incorporation of metal centers into the C₃N₄ molecular skeleton. Taking into consideration the low mass loading of 0.153 mg/cm² of the composites, the C_(dl) values were also comparable to result reported in prior studies (see Table 2 above).

In these implementations, electrochemical impedance measurements were then carried out to quantify the corresponding R_(ct). FIG. 23 depicts the typical Nyquist plots of C₃N₄—Ru—F at various overpotentials, and R_(ct) was estimated by fitting the data to Randle's equivalent circuit (see FIG. 4E inset). The figure illustrates that R_(ct) decreased significantly with increasing overpotentials. FIG. 4E compares the Nyquist plots of the various electrocatalysts at the overpotential of −50 mV, where R_(ct) was estimated to be 285Ω for C₃N₄—Ru—F, 658Ω for C₃N₄—Ru—P and 1550Ω for C₃N₄. The figure confirms that the embedding of ruthenium metal centers within the C₃N₄ matrix greatly facilitated the electron-transfer kinetics, which is consistent with results from the above voltammetric measurements.

In addition to excellent electrocatalytic activity, stability of the catalysts is an important variable in practical applications. For C₃N₄—Ru—F, the polarization profiles remained almost invariant after 1,000 cycles of potential scans, with the η₁₀ value increased by only 3 mV, suggesting long-term durability of the catalyst (see FIG. 4F). XPS measurements showed no variation of the C 1s and Ru 3d electrons after 1,000 electrochemical cycles (see FIG. 24).

In these implementations, DFT calculations were conducted to examine the effect of the incorporation of ruthenium ions into the C₃N₄ matrix on the band structures and Gibbs free energy of hydrogen adsorption and reduction. 2×2 cells of C₃N₄ and C₃N₄—Ru were used for the calculations (see FIG. 25-27), where a ruthenium ion was bonded to two nitrogen sites, as suggested by experimental results (see FIG. 2, and Table 1 above). The calculated band structure of C₃N₄ (see FIG. 5A) suggests an indirect band gap of about 1.3 eV, which is in general agreement with the PDOS plot in FIG. 5B. In contrast with the semiconducting nature of C₃N₄, the band structure of C₃N₄—Ru (see FIG. 5C) shows no band gap, which is attributable to the embedding of ruthenium ions into the C₃N₄ matrix caused a charge transfer between C₃N₄ and ruthenium ions, as observed in XPS measurements (see FIG. 2). Additionally, the PDOS profile of C₃N₄—Ru shows a large density of states at the Fermi level, with the primary contributions from the Ru 4d and 5s orbitals (see FIG. 5D). This indicates that the incorporation of ruthenium ions into C₃N₄ led to redistribution of the electrons within the composite, crossing of the Fermi level with the conduction band, and hence enhanced charge mobility and electrical conductivity, consistent with electrochemical impedance measurements (see FIG. 4E). This may also explain the quenching of the C₃N₄ PL emission (see FIG. 3).

This discrepancy of the electronic structures of C₃N₄—Ru, as compared to that of C₃N₄, is likely responsible for the enhanced electrocatalytic activity of C₃N₄—Ru. HER typically involves a three-state process, an initial H⁺ state, an intermediate H* state, and ½H₂ state as the final product (see FIG. 5E), and the Gibbs free-energy of the formation of the intermediate H* state, |ΔG_(H*)|, can be used as the descriptor of the HER performance for different electrocatalysts. For an ideal HER electrocatalyst, |ΔG_(H*)| should be zero. In the disclosed embodiments, C₃N₄ was found to exhibit a Gibbs free energy of ΔG^(C) _(H*)=+1.23 eV and ΔG^(N) _(H*)=−0.63 eV for the carbon and nitrogen bonding sites (see left inset to FIGS. 5E and 26), respectively. Yet, when ruthenium ions were incorporated into the C₃N₄ matrix, the |ΔG_(H*)| values were found to be substantially lower at the Ru, C, and N binding sites (see right inset to FIGS. 5E and 27), ΔG^(C) _(H*)=−0.48 eV, ΔG^(N1) _(H*)=+0.57 eV, ΔG^(N2) _(H*)=+0.60 eV and ΔG^(Ru) _(H*)=−0.49 eV, suggesting enhanced hydrogen adsorption by ruthenium ion complexation to C₃N₄. This is also illustrated by FIG. 5F, which depicts the interfacial charge transfer between C₃N₄ and ruthenium ions (e.g., by computing the charge density difference between C₃N₄—Ru and C₃N₄+ isolated Ru atom) and the resulting charge redistribution among the entire cell. The incorporation of ruthenium ions into the C₃N₄ molecular skeleton drastically enhanced the electrical conductivity, and facilitated the adsorption of hydrogen to various binding sites in the composites, which is generally responsible for the enhanced HER performance (see FIG. 27).

In certain implementations of the disclosed technology, the HER performance is further enhanced by incorporating rGO into the ruthenium ion-complexed carbon nitride nanosheets, forming C₃N₄-rGO-Ru complex. The incorporation of Ru ions led to a change of the electronic structure of the material due to charge transfer from C₃N₄ to Ru ions, as confirmed by XPS measurements, increase of electrochemical surface area, electric conductivity and charge carrier density, decrease of charge transfer resistance and narrowing of the material bandgap. The results included a dramatic enhancement of the electrocatalytic activity towards HER, with a low η₁₀ of only −80 mV, a Tafel slope of 55 mV/dec, and exchange current density of 0.462 mA/cm².

The synthesis of the C₃N₄-rGO-Ru complex is schematically illustrated by FIG. 10. In these implementations, GO was reduced by refluxing well-dispersed C₃N₄ in aqueous solution with ascorbic acid to form C₃N₄-rGO composites. Then a calculated amount of RuCl₃ was added into the solution to form C₃N₄-rGO-Ru complexes. The formation of the composites was first confirmed by AFM measurements and the results were depicted in FIGS. 11a-c . As illustrated by FIG. 11a ₁, the thin-layer C₃N₄ is well-dispersed without any aggregation. FIG. 11a ₂ exhibits the representative heights of the C₃N₄ sheets marked by the line in FIG. 11a ₁, and it was found that the average height of the C₃N₄ sheets is about 2.0±0.4 nm (FIG. 11a ₃) by counting more than 100 sheets which is consistent with previous study, indicating a 4-7 layers C₃N₄(C₃N₄ possesses the layer distance of about 0.33 nm). After the introduction of rGO into C₃N₄ sheets, it was found that the products became agglomerations (see FIG. 11b ₁), and the heights at different positions of the marked line are also varied from 2.2, 7.0, 8.4, to 10.3 nm (see FIG. 11b ₂) while the average thickness were about 6.3±3.7 nm counting by more than 100 positions (see FIG. 11b ₃). This indicates that the C₃N₄ sheets were assembled onto the rGO and formed a sandwich structure, leading a strong interaction between C₃N₄ and rGO because of the interlayer π-π interaction, van der Waals force and hydrogen bonding of oxygen-containing groups. After the incorporation of ruthenium ions, the AFM topograph of the C₃N₄-rGO-Ru complexes exhibited similar results (see FIG. 11c ) as C₃N₄-rGO composites where the heights are varied from 2, 6.2 to 9.5 nm (see FIG. 11c ₂) based on the marked line in FIG. 11c ₁ and the thickness distribution is about 7.3±4.0 nm counting by more than 100 positions (see FIG. 11c ₃). This suggested that the incorporation of ruthenium ions into the C₃N₄ matrix has no effect on the morphology of the materials (though it may have significant effect on the electronic structure). FIGS. 11d-e depict the TEM images of C₃N₄-rGO composites and C₃N₄-rGO-Ru complexes and their corresponding elemental maps. The similar agglomeration structures of C₃N₄-rGO composites and C₃N₄-rGO-Ru complexes were observed and all the elements were well distributed, while the Ru was not detected in C₃N₄-rGO composites which was presented in C₃N₄-rGO-Ru complexes.

To further confirm the existence of these elements and study their valance states, X-ray photoelectron spectroscopic (XPS) measurements were carried out in these implementations and the results are illustrated by FIG. 12, and Table 3 below.

TABLE 3 Atomic percentages of each elements in the materials based on XPS measurements. C N O Ru C₃N₄ 42.14% 53.86% 4.00% — C₃N₄-rGO 61.67% 16.65% 21.68% — C₃N₄-rGO-Ru 67.88% 7.48% 22.71% 1.93%

As illustrated by the full spectra in FIG. 12a , the peaks at 284, 399, and 531 eV may be assigned to the C 1s, N 1s and O 1s, respectively, suggesting the existence of these elements in C₃N₄, C₃N₄-rGO, and C₃N₄-rGO-Ru. In addition, the peak at 464 eV, which is absent in the first two samples, can be ascribed to Ru 3p electrons, while the Ru 3d peaks were overlapped with Cis. Based on integrated peak areas, the contents of each element in C₃N₄-rGO-Ru samples were estimated to be 67.88% for C, 7.48% for N, 22.75% for O and 1.93% for Ru. In these implementations, high-resolution XPS spectra in the range of Ru 3d, C 1s (see FIG. 12b ) and N 1s (see FIG. 12c ) were compared. Two apparent peaks at 281.96 and 286.16 eV were deconvoluted, which were attributed to Ru 3d_(5/2) and Ru 3d_(3/2) of Ru(II)-N moieties embedded in N-GQD or C₃N₄. As for the carbon signals, C₃N₄ shows a major peak centered at 287.4 eV due to the sp2-hybridized carbon moieties (N—C═N) embedded in C₃N₄ matrix as well as a minor one at 284.2 eV from defective carbon. F or C₃N₄-rGO and C₃N₄-rGO-Ru, the N—C═N peak positively shifted to 287.85 and 288.02 eV, respectively, indicating reduced electron density due to the charge delocalization from C₃N₄ to rGO and/or Ru complex centers. In addition, there were two new peaks at 284.31 eV and 286.16 eV for both C₃N₄-rGO and C₃N₄-rGO-Ru due to C═C and C—O bonds of rGO, respectively. They are relatively smaller than rGO where C═C at 284.6 eV and C—O at 286.5 eV, suggesting an increase of electron density. Similarly, the N signals can be fitted by two peaks, where the one at 397.86 eV is from sp²-hybridized pyridinic nitrogen moieties (C—N═C) and the other one at 399.62 eV from sp³-hybridized tertiary nitrogen moieties (N—(C)₃). In comparison with C₃N₄, there was an apparent positive shift for both C₃N₄-rGO and C₃N₄-rGO-Ru. For instance, the C—N═C signal of C₃N₄ was located at 397.86 eV, which shifted to 398.47 eV and 398.65 eV for C₃N₄-rGO and C₃N₄-rGO-Ru, respectively. The positive shifts may be attributed to the efficient charge transfer from C₃N₄ to rGO and/or Ru centers. For the C₃N₄-rGO-Ru sample, the atomic ratio of C—N═C to Ru is about 2.3, suggesting that the Ru centers are coordinated by about 2 C—N═C sites. Overall, the XPS measurement reflected the strong interaction among each part of C₃N₄-rGO-Ru and resulted in the electron redistribution, which may have an advantageously positive impact on the electrocatalytic performance of the materials.

In these implementations, the electrocatalytic activities towards HER were compared by electrochemical measurements in N₂-saturated 0.5 M H₂SO₄ electrolyte. FIG. 13a illustrates the typical polarization curves of the various electrocatalysts loaded onto a glassy carbon electrode, from which η₁₀ was estimated to be −416, −206, −80, and −41 mV for C₃N₄, C₃N₄-rGO, C₃N₄-rGO-Ru, and Pt/C respectively. Correspondingly, the Tafel plots of these electrocatalysts were derived from polarization curves. As illustrated by FIG. 13b , the Tafel slope was 125, 82, 55, and 32 mV/dec for C₃N₄, C₃N₄-rGO, C₃N₄-rGO-Ru, and Pt/C, respectively. FIG. 14 further depicts the exchange current densities (J₀) of various catalysts and J₀ of C₃N₄-rGO-Ru is 0.462 mA/cm², which is very close to that of Pt/C (J₀=0.576 mA/cm²) and is 12.8 times that of C₃N₄-rGO (J₀=0.036 mA/cm²) and 103 times that of C₃N₄ (J₀=0.0045 mA/cm²). Whereas the state-of-the-art Pt catalyst possessed the best HER performance, the C₃N₄-rGO-Ru composite displayed the second best electrocatalytic activity towards HER. The Tafel slope between 40 and 120 mV/dec also signified the rate-determining step of hydrogen evolution on the catalysts is Volmer-Heyrovsky reaction. Furthermore, the C₃N₄-rGO-Ru showed a large cathodic current density of 35.83 mA/cm² which was 14.6 times that of C₃N₄-rGO (2.45 mA/cm²) and 227 times of C₃N₄ (0.158 mA/cm²) at the overpotential of −150 mV (see FIG. 15). This suggests that the HER performance was dramatically enhanced with the introduction of rGO and Ru ions into the composites. FIG. 13c and Table 4 below compared the η₁₀ and Tafel slope of several carbon-based HER electrocatalysts reported in recently years. The disclosed C₃N₄-rGO-Ru composite is generally superior to these examples.

TABLE 4 The HER electrocatalytic performance of the disclosed C₃N₄-rGO- Ru complexes (first row) with prior carbon-based electrocatalysts (subsequent rows labelled a-n). Tafel η₁₀ slope Jo Loading Cal catalyst (mV) (mV/ dec) (μA/cm²) (mg/cm²) (mF/cm²) −80 56 462 0.153 27.8 a BCN −70 100 51 0.943 0.108 b PCN/NG −80 49.1 430 0.57 N/A c S, N—C −116 68 N/A 0.285 27.4 d Co—C—N −138 55 N/A N/A 400 e C₃N₄—Ru −140 57 72 0.153 18.4 f N—P—C −163 89 160 0.159 N/A g CN-nrG −207 54 39.8 0.143 N/A h C₃N₄@G −219 53 49 0.142 10.6 i p-WMCNT −220 71.3 16 0.36 N/A j NENU −237 96 36 N/A N/A k C₃N₄-NG −240 51.5 0.35 0.1 5.0 1 NS-500 −276 81 8.4 N/A N/A m C₃N₄—Cu −390 76 N/A 0.28 N/A n p-N, P-G −420 91 0.24 0.204 10.6

In these implementations, electrochemical impedance measurements were carried out to quantify the corresponding charge-transfer resistance (R_(ct)). FIG. 16 illustrates the representative Nyquist plots of C₃N₄-rGO-Ru complexes at various overpotentials and R_(ct) decreased significantly with the increase of overpotentials. FIG. 13d compares the Nyquist plots of the catalysts at the overpotential of −100 mV. By fitting the data to Randle's equivalent circuit (see inset to FIG. 13d ), R_(ct) was estimated to be 1366, 157, and 20Ω for C₃N₄, C₃N₄-rGO, and C₃N₄-rGO-Ru. C₃N₄ is a semiconducting material and the introduction of rGO can obviously reduce R_(ct). In embodiments where Ru ions were coordinated into C₃N₄-rGO, the R_(ct) of the C₃N₄-rGO-Ru was further reduced, signifying that the coordination of Ru centers within the composite can efficiently facilitate the electron-transfer kinetics and thus reducing the overpotential of HER.

Another factor that may affect the HER performance is the number of active sites. In C₃N₄—Ru, it was found that the electron redistribution caused by the coordination of Ru can facilitate the adsorption of hydrogen and provide more active sites. In the disclosed embodiments, the active sites of various electrocatalysts were compared by their electrochemical double-layer capacitance (C_(dl)) as it is proportional to effective surface area. FIG. 17a illustrates the representative CV curves of C₃N₄-rGO-Ru composite (the CV curves of C₃N₄ and C₃N₄-rGO are illustrated by FIG. 18) collected at the scan rates of 10 to 60 mV/s in the non-faradaic reaction range. As illustrated by FIG. 17b , the C_(dl) of C₃N₄-rGO-Ru was estimated to be 27.8 mF/cm², which is about 7.1 times that of C₃N₄ (3.9 mF/cm²) and 3.3 times that of C₃N₄-rGO (8.4 mF/cm²). This is consistent with the HER performance that the C₃N₄-rGO-Ru possessed the best activity among the series. This dramatic enhancement may be attributed to the enhanced electrical conductivity of the composite with the incorporation of rGO and/or Ru centers in to C₃N₄. To reveal the electrical conductivity of the materials, the Nyquist plots (see FIG. 17c ) were collected again without the addition of carbon black into the ink. There is an apparent R_(s) of 35.4Ω for C₃N₄, while it reduced to 10.0Ω when rGO was introduced and further reduced to 7.9Ω when formed a C₃N₄-rGO-Ru composite. This confirmed that the enhancement of HER performance is accounted for the enhanced electrical conductivity. A Mott-Schottky analysis was performed and the Mott-Schottky plots of various materials are shown in FIG. 17d . The Mott-Schottky plots exhibited a positive slope, indicating n-type characteristic nature of these C₃N₄-based materials. Additionally, the flat-band potential (E_(fb)) of C₃N₄ was derived to be −1.35 V (vs. Ag/AgCl), while the other two anodic shifted to −0.95 V. Thermodynamically, E_(fb) of a n-type semiconductor determines the conduction band position, and the positive shift of the conduction band narrows the energy barrier of hydrogen evolution (H+/H₂-0.59 V vs. Ag/AgCl at 0.1 M Na₂SO₄) and thus enhances the electrocatalytic performance of the material. Furthermore, the charge carrier densities of the materials can be calculated by the following equation: N_(d)=2/eεε₀m, where N_(d) is the charge carrier density, e is the elementary charge of an electron, ε is the dielectric constant (≈10 in the present study), Co is permittivity in vacuum (8.85×10⁻¹² F/m), and m is the slope of the Mott-Schottky plot. The charge carrier densities were evaluated to be 3.67×10¹⁵, 1.42×10¹⁷, and 9.05×10¹⁷ cm⁻³, for C₃N₄, C₃N₄-rGO, and C₃N₄-rGO-Ru, respectively, suggesting that the charge carrier density of the C₃N₄-rGO-Ru is 247 times of that C₃N₄ and 6.4 times of that C₃N₄-rGO. The Mott-Schottky analysis is highly consistent with the EIS measurement of the electrocatalytic performance.

The introduction of ruthenium ions into the composites of C₃N₄-rGO to form C₃N₄-rGO complexes by the strong coordination reaction between ruthenium ions and carbon nitride. The disclosed implementations exhibited an enhanced electrocatalytic activity towards hydrogen evolution reaction, which possessed an overpotential of −80 mV for a 10 mA/cm² current density, Tafel slope 55 mV/dec, exchange current density 0.462 mA/cm². This significant enhancement is attributed to electron redistribution after the introduction of rGO into C₃N₄ and the incorporation of Ru ions. These implementations resulted in efficient narrowing of bandgap of the materials, enhancement of the electric conductivity and charge carrier density, increase of active sites, and reduction of the charge transfer resistance.

Having described and illustrated the principles of the invention with reference to illustrated embodiments, it will be recognized that the illustrated embodiments may be modified in arrangement and detail without departing from such principles, and may be combined in any desired manner. And although the foregoing discussion has focused on particular embodiments, other configurations are contemplated.

Consequently, in view of the wide variety of permutations to the embodiments that are described herein, this detailed description and accompanying material is intended to be illustrative only, and should not be taken as limiting the scope of the invention. What is claimed as the invention, therefore, is all such modifications as may come within the scope and spirit of the following claims and equivalents thereto. 

What is claimed is:
 1. A method, comprising: incorporating graphene oxide (GO) in a solution; reducing the graphene oxide (GO) by refluxing carbon nitride (C₃N₄) in the solution to form carbon-nitride refluxed-graphene-oxide (C₃N₄-rGO) composites; and incorporating ruthenium ions into the C₃N₄-rGO composites to form C₃N₄-rGO-Ru complexes.
 2. The method of claim 1, further comprising using the C₃N₄-rGO-Ru complexes as a catalyst in a hydrogen evolution reaction (HER) process.
 3. The method of claim 1, further comprising assembling C₃N₄ sheets onto the refluxed GO to form a sandwich structure.
 4. The method of claim 1, wherein the reducing includes using ascorbic acid in the solution.
 5. The method of claim 1, wherein the incorporating includes refluxing ruthenium chloride (RuCl₃) in the solution.
 6. The method of claim 1, further comprising confirming the formed C₃N₄-rGO composites by atomic force microscope (AFM) measurements.
 7. The method of claim 1, further comprising performing x-ray photoelectron spectroscopic (XPS) measurements on the C₃N₄-rGO-Ru complexes.
 8. The method of claim 1, further comprising performing transmission electron microscopy (TEM) on the C₃N₄-rGO-Ru complexes.
 9. The method of claim 1, wherein the solution is an aqueous solution.
 10. The method of claim 9, wherein the aqueous solution is water. 